The invention relates to an ionisation chamber for ion beams and to a method of monitoring the intensity of an ion beam according to the preamble of claims 1 and 16.
Such ionisation chambers are also known in the prior art as ion counting tubes. The chamber housing is customarily made from a tube, with one of the two end portions of the tube serving as the beam inlet window and the other end of the tube serving as the beam outlet window. The tube is filled with a counting gas under reduced pressure and has a cylindrical high-voltage cathode that lies coaxially in the counting tube, insulated from the tube wall. Located in the centre of the tube is a cylindrical high-voltage anode which is insulated from the high-voltage cathode and the surrounding tube. In order to operate the ionisation chamber, a voltage is applied between the high-voltage cathode and the high-voltage anode and the current between the cathode and anode is measured. If charged particles, such as ions, are passed through the ionisation chamber or are captured by the ionisation chamber, the current between the cathode and the anode increases in dependence upon the number of ions that pass through the ionisation chamber. More complex cylindrical ionisation chambers have a large number of axially aligned anodes in order, for example, to measure the path of a charged particle or ion through a tubular ionisation chamber by means of anodes distributed axially over the cross-section.
A disadvantage of such cylindrical ionisation chambers is their large axial dimension and the relatively complex construction of the counting anodes. Moreover, the space required by such ionisation chambers in the direction of the diffusion of a beam is relatively large. The space available at the beam outlet in a treatment room in front of patients is, however, very limited. Moreover, with therapy systems in which an ion beam is scanned over the entire extent of a tumour tissue, there must be available an ionisation chamber of hitherto unknown dimensions in terms of breadth and length. Generally, all the measurements in the beam must be carried out in front of the patient in the transmission mode. It is imperative to avoid impairment of the quality of the beam, for example as a result of projectile fragmentation and angular scatter of the beam particles.
The problem underlying the invention is accordingly to provide an ionisation chamber for ion beams and a method of monitoring the intensity of an ion therapy beam that overcomes the disadvantages of existing ionisation chambers, is suitable for monitoring and controlling patient irradiation in the context of tumour therapy with heavy ions that are concentrated with high energy into a pencil beam, in which the dimensions of the detector in the direction of the beam are small, that enables a high level of safety to be achieved, especially in respect of plasma and spark formation, and can be used in the field of medicine.
For that purpose, the ionisation chamber for ion beams consists of a chamber housing, a beam inlet window and a beam outlet window, a chamber volume filled with counting gas and a high-voltage anode and a high-voltage cathode. The ionisation chamber is constructed flat and sandwich-like from plate-shaped large-surface-area structures of those components, which are aligned orthogonally relative to the axis of the ion beam. A centrally arranged large-surface-area orthogonally aligned plate-shaped counting anode is surrounded on both sides by a large-surface-area plate-shaped high-voltage cathode consisting of two parallel cathode plates. The chamber housing consists of a housing frame which frames a virtually square ionisation chamber volume, and on which frame the beam inlet window and beam outlet window are mounted gas-impermeably. Such a device has the advantage of being easier to maintain since the plate-shaped construction can be removed from the housing frame and replaced by simply dismantling or removing different plate structures. The plates can be replaced easily and suitable numbers thereof can be held in stock. The plate-shaped structure also enables mass production of spare parts and finished ionisation chambers.
In a preferred embodiment of the invention, the counting gas is a gas mixture of argon or krypton and carbon dioxide, preferably having a gas volume mixing ratio of 4:1, which is adapted to the energy and intensity of the ion beam, and is introduced into the ionisation chamber. A counting gas of such composition has the advantage, compared with customary air-filled cylindrical ionisation chambers, that the measurements are easier to reproduce since in this case air humidity does not influence the sensitivity of the ionisation chamber. Such a counting gas ensures a good signal/noise ratio and makes available a high dynamic range in the particle rates. With the preferred counting gas, sufficient dielectric strength is also ensured.
Such a counting gas is preferably of the highest purity since the signal sensitivity, especially in its amplitude and waveform, is impaired by impurities. Moreover, inside the chamber volume there are preferably used for the individual plate elements and other supporting and insulating elements as well as for auxiliary units and sensors materials that do not release gases, or elements and components that do release gases are cast in epoxy resin.
In a further preferred embodiment, the ionisation chamber has sensors that are mounted in the housing frame in through openings that are gas-impermeably sealed and that measure the counting gas pressure and the counting gas temperature. The ionisation chamber is operated with slightly elevated pressure compared with the ambient air, which advantageously makes penetration of extraneous gases more difficult. For that purpose, the extent of the gas reflux from the chambers is monitored by a sensor system in the counting gas outlet region or in the outlet. By measuring gas pressure and gas temperature, it is advantageously possible to monitor the gas density and, if necessary, keep it constant, the gas density being used directly in the determination of the ion beam particle number.
The beam inlet window and the beam outlet window, which are substantially square, preferably consist of radiation-resistant non-polarisable plastics films. These are secured to metal plate-shaped frames, which in turn seal off the ionisation chamber volume gas-impermeably from the beam inlet window and from the beam outlet window by means of O-ring seals in the housing frame. That gas-impermeable construction keeps impurities away from the counting gas and gas exchange of the chamber volume with the environment by diffusion is minimised even when the ionisation chambers are not in operation.
The beam inlet window and the beam outlet window preferably comprise polyimide or polyester films, which has the advantage that exclusively radiation-resistant and non-polarisable materials are exposed to the ion beam, so minimising the effect on the ion beam and the ion beam intensity.
In a further preferred embodiment of the invention, the beam inlet window and the beam outlet window are metallised on the side facing the ionisation chamber volume. Such metallisation of the beam inlet window and the beam outlet window prevents the windows from becoming charged and thus prevents falsification of the measurement values, since charges can be conducted away to the ionisation chamber housing directly by way of the metallisation of the windows and by way of the window frames. The ionisation chamber housing is thus advantageously earthed.
Such metallisation can be achieved by aluminising or nickel-plating one of the sides of the beam inlet window or beam outlet window. Such aluminised films have a conductive layer in order to avoid high electrical field densities as trigger points for gaseous discharges, so that the occurrence of gaseous discharges is minimised. Moreover, metallised films form a smooth surface which also serves to prevent high electrical field densities at trigger points for gaseous discharges.
The large-surface-area plate-shaped counting anode and the large-surface-area plate-shaped high-voltage cathode preferably consist of mesh mounted in a frame which is supported against the housing frame in an electrically insulating manner. In the process, electrically insulating spacing elements define the spacing between the counting anode and the high-voltage electrode. The use of meshes instead of films for the anode and cathode has the advantage that it is possible to operate with relatively large mechanical pre-stressing for the detector planes of mesh material. The uniformity of the signal across the site and across the extent of the detector surface of the ionisation chamber is thus improved, which has an advantageous effect in particular in the case of the relatively large chamber cross-sections in the edge regions of the active volume of the chambers.
Compared with large-surface-area films as counting anode surfaces or as cathode surfaces, meshes have the advantage that there is less sagging, in particular when high voltage is applied, because of their higher pre-stressing capacity. Such sagging is caused by mutual electrostatic attraction of the films or mesh electrodes mounted parallel with one another. When meshes are used, however, the spacing of the electrodes from one another, especially in the centre, remains relatively constant so that the field densities can advantageously be kept locally constant.
Instead of using metal fibre mesh, it is preferable to use for the large-surface-area plate-shaped counting anode and the large-surface-area plate-shaped high-voltage cathode mesh made from metal-coated plastics fibres. Such mesh made from composite fibres of plastics and metal coating has the advantage that it is lighter and can be loaded with relatively high pre-stressing and it has a low nuclear charge number for the carrier filaments. The electrode function is provided by the metal coating and a capacity to bear a high mechanical load is achieved as a result of the plastics core of the fibres, that capacity to bear a high mechanical load in turn being a precondition for high mechanical pre-stressing of the high-voltage plate electrodes.
The large-surface-area plate-shaped high-voltage electrode and the large-surface-area plate-shaped counting anode are preferably made of nickel-coated plastics mesh or nickel-coated polyester mesh. That composite material has the advantage not only that its plastics part consists of radiation-resistant and non-polarised materials but also that it provides a smooth surface, which allows high electrical field densities without triggering gaseous discharges.
In a preferred embodiment, the counting gas is supplied into the region of the ionisation chamber volume that is lowermost with respect to gravity, and is discharged in the uppermost region. For that purpose, the housing frame of the ionisation chamber has a counting gas inlet opening and a counting gas outlet opening. By means of that preferred embodiment, advantageously a laminar flow of counting gas through the ionisation chamber can be provided by way of the counting gas inlet opening and the counting gas outlet opening. Inside the chamber, the counting gas is preferably guided through stainless steel tubes having variable outlet and inlet holes.
A gas flow sensor is preferably arranged outside the ionisation chamber volume in order to monitor the throughflow of counting gas, so as to keep the chamber volume as small as possible. In addition to simply monitoring the counting gas throughflow, it is also possible to regulate the counting gas with the aid of such gas flow sensors in conjunction with pressure and temperature sensors.
Preferably the central counting anode and the high-voltage cathode in question are arranged relative to one another at a spacing of from 3 to 13 mm, especially 5 mm, and can be operated at high voltages of more than 1500 V. For that purpose, the plate-shaped electrodes must be insulated from one another by insulating parts, such as the frame, the spacing pieces, and adhesion and casting compositions, those insulating parts having high volume and surface electrical resistance values of, respectively, from 1012 to 1014 xcexa9/cm3 and from 1016 to 1018 xcexa9/cm. This advantageously reduces leakage or tracking currents, which would otherwise falsify the measurement and impair the sensitivity of the system as a whole.
In a further preferred embodiment, the ionisation chamber is developed to form an ionisation chamber system for ion beams. For that purpose, a plurality of ionisation chambers of the type according to the invention are arranged behind one another in the direction of the beam and are used to form a system for monitoring the intensity of a heavy ion therapy beam. Owing to the high safety standards that must be met in the case of therapy beams, at least two ionisation chambers are arranged in tandem behind one another in the direction of the ionisation beam in order to monitor the individual dose of a volume element and the layer dose of a scanned layer and are used for the independent monitoring of the total dose of a treatment cycle independently of the ionisation chamber for monitoring the individual dose and the layer dose.
Owing to the flat construction of a single ionisation chamber, that ionisation chamber system has the advantage that in the direction of the beam it requires very little space whereas, transverse to the beam, it extends over the full scanning surface. Since using a raster scanner the ion beam scans the target volume by volume and by layer, advantageously the individual dose per volume element is monitored by a first ionisation chamber in the ionisation chamber system and the layer dose is monitored by adding together all the individual doses of a scanned layer. A second ionisation chamber independent of the first ionisation chamber can advantageously monitor the total dose of a treatment cycle. Instead of the preferred two ionisation chambers, it is also possible to connect three ionisation chambers behind one another, which then monitor the individual dose of a volume element, the layer dose of a scanned layer and the total dose of a treatment cycle, respectively.
In a further preferred embodiment, the first and second ionisation chambers monitor the pixel dose and the layer dose with redundancy. The second ionisation chamber thus monitors the first ionisation chamber. A third ionisation chamber is connected to different electronics and monitors integral values, such as when the dose falls short of a maximum dose in the treatment plan.
In order to increase safety, the individual dose for a volume element can be measured in the first and second ionisation chambers of an ionisation chamber system comprising three ionisation chambers, and the results of the first and second ionisation chambers can be compared so that if the measured data from the first and second ionisation chambers depart from a predetermined tolerance range, a rapid switch-off of the ion therapy system can be triggered. Such a comparison increases the operating safety of the first and second ionisation chambers. Similarly, the layer dose of a layer to be irradiated (also called irradiation layer) can be measured and compared by the second and third ionisation chambers of the ionisation chamber system comprising three ionisation chambers so that if the measured results from the second and third ionisation chambers exceed a predetermined tolerance range a safety switch-off of the ion beam therapy system can be triggered.
The method of monitoring the intensity of a heavy ion beam by means of ionisation chambers or by means of the ionisation chamber system comprises the following method steps:
a) measurement of the intensity dose of an irradiation volume element of a planned irradiation raster for an irradiation layer by means of a first ionisation chamber;
b) monitoring of the measured value of the intensity dose of an irradiation volume element by a second ionisation chamber arranged in tandem behind the first ionisation chamber;
c) comparison of the measured value of the first ionisation chamber with the monitoring value of the second ionisation chamber and clearance for the irradiation of the next irradiation volume element of a planned irradiation raster of an irradiation layer when the two irradiation intensity values match within a predetermined desired value range;
d) emergency switch-off of the radiation treatment when the predetermined desired value range is exceeded and readjustment of the intensity when it falls short of the predetermined desired value range;
e) repetition of the steps for the subsequent planned treatment layers until the volume of tissue to be irradiated has been fully scanned;
f) integration of all the measured radiation doses of the monitoring values in a third ionisation chamber, which is arranged in tandem behind the first and second ionisation chambers, in order to monitor the total radiation to which a volume of tissue is subjected during a treatment cycle.
By means of that method, advantageously the number of particles or heavy ions extracted per second from an accelerator and used as an ion beam for tumour therapy is measured. The number of particles is subject to large temporal variations and must accordingly be measured in real time during irradiation directly by those ionisation chambers. The current measured at the outlet of the chambers is proportional to the ion beam current when the particle energy remains constant. In typical beam currents of the accelerator, the currents coming from the ionisation chambers are in the region of xcexcA.
The response speed of the ionisation chambers is limited by the drift time of the ionised counting gas molecules in the ionisation chamber and has a delay constant in the order of magnitude of about 10 xcexcs. Measuring electronics convert the current from the ionisation chamber to a proportional frequency of pulses. For that purpose, voltage signals are produced from the current signal and pulses are formed from the voltage signal by means of amplitude-frequency conversion, the frequency of which pulses is proportional to the voltage. Accordingly, a pulse corresponds to a specific charge produced in the ionisation chamber, the charge in turn being produced by a specific particle number of the ion beam. The number of pulses produced is thus proportional to the number of ions flowing through the ionisation chamber.
Accordingly, using the method it is advantageously possible to monitor the intensity dose of an irradiation volume element, the intensity dose of a total irradiation layer and finally the intensity dose of a whole treatment cycle. The method also comprises safety-relevant redundancy, in that in practice both the intensity dose of an irradiation volume element and the intensity dose of an irradiation layer are measured in two ionisation chambers and the measured values are compared directly with one another, so that in the event of unacceptable deviations an emergency switch-off of the system can be triggered. It is also possible to add to each of the three ionisation chambers summation electronics so that all three ionisation chambers can monitor the total dose of a radiation cycle of an irradiation volume simultaneously and in parallel. Thus, by means of the ionisation chambers according to the invention and in particular by means of the ionisation chamber system according to the invention comprising three ion beam chambers arranged behind one another in the direction of the ion beam, it is possible to achieve the greatest possible safety and reliability during the irradiation of a tumour volume with ion beams.
In a preferred embodiment of the method, the order in which the method steps are carried out is adapted optimally to the planned treatment cycle in question. In a preferred further development of the method, the monitoring function of the first and second ionisation chambers is provided by a single ionisation chamber. Whilst this reduces the redundancy of the method, the space required for the detector systems in the form of ionisation chambers is advantageously reduced.